To improve fuel economy automotive air conditioners have been provided with a clutch to avoid running the compressor when further cooling is not desired. Clutch operation has been controlled by thermostatic or pressure responsive switches with the pressure switch generally being used only with flooded evaporator systems.
Pressure switches used in the past use a single spring for determining the trip and reset points and require calibration. They make contact slowly and a snap-disc is used to make and maintain positive electrical contact. Small chips can break off and fall into the contact area as the calibration screw is turned against the plastic housing. The chips contaminate the contacts and impair function and service life. The snap-disc introduces variation into the calibrated setting.